Formation of polished surfaces in natural rocks: experimental and field constraints

Mirror-like Surfaces (MSs) are ultra-polished fault surfaces that reflect visible light, thanks to their sub-micrometre-scale surface roughness. They are often found in carbonate-hosted seismogenic fault zones, however, their formation mechanism is still debated. Experiments show that MSs can develop both under seismic (slip rate ≈1 m/s) and sub-seismic (slip rate ≈0.1-10 μm/s) deformation conditions, involving various physical-chemical processes operating over a broad range of pressure-temperature conditions, strain and strain rates. To constrain the formation mechanism and the role of MSs in the seismic cycle, I investigated MSs from faults cutting bituminous dolostones in the Italian Central Apennines (Monte Camicia Thrust Zone, measured displacement from few mm to few m) using a combination of field and microstructural observations, shear and quasi-static loading laboratory experiments, thermal maturity and roughness analysis. I found that (1) these MSs form in faults/fractures also with negligible displacement and, (2) slip zones bounded by a MS record the main phases of the seismic cycle. In the studied faults, aseismic slip occurs via cataclasis, pressure solution and viscous flow in bitumen. Instead, the presence of fragments of older slip zones and of organic matter-rich clasts cemented by calcite suggests the occurrence of multiple seismic ruptures followed by calcite precipitation. To investigate how the deformation conditions (i.e., water saturation, quantity of organic matter, slip rate) affect the frictional behaviour and the formation of MSs, I deformed bituminous dolostones gouges at seismic (V =1-3 m/s) and sub-seismic (V =15-100 μm/s) slip rates under room-humidity and fluid-pressurized conditions. In the experiments, MSs were always obtained at seismic slip rates while at sub-seismic slip rates, MSs did not form under fluid-pressurized conditions when organic matter in the gouge was <20-25%. MSs are associated with strain localization and composed of a few μm-thick layer of nanoparticles (<50 nm in size) arranged in aggregates (“seismic” MSs) or nanofibers (“sub-seismic” MSs). The different arrangement of nanoparticles may serve as an indicator of the seismic vs. sub-seismic formation conditions of the MSs. In addition, I conducted biomarker thermal maturity analyses on bitumen smeared on natural and experimental MSs, to investigate whether MSs formed during the coseismic phase (linked to frictional heat pulses) or during the inter-seismic period (which lacks heat pulses). With respect to the associated slip zone, higher thermal maturity was measured in some of the natural MSs and in the experimental MSs obtained at seismic slip rates, suggesting that bitumen biomarkers can record temperature increase due to frictional heat pulses associated with seismic slip. Finally, to investigate whether MSs may results from quasi-static-processes typical of the inter-seismic phase and in absence of slip displacement, I conducted quasi-static loading experiments on limestone stem with imposed initial roughness lasting from 10 s to 120 h in presence or absence of pressurized water. Hundreds of <1 mm2 in size MS patches formed on the sample surfaces, demonstrating that MSs can form also in absence of slip along pre-existing surfaces. PSD roughness analysis of these pre- and post- experiment surfaces, and of natural MSs in bituminous dolostones with <1 m slip, revealed that MSs are not self-affine at all scales, with a main discontinuity at wavelengths of ≈10 μm. Scaling properties (Hurst exponent “H”) of experimental surfaces for wavelengths <10 μm, are influenced by slip (H increases) and quasi-static processes like asperities flattening (H decreases) and pressure-solution (H increases).